Pertussis toxin (PTX) is a complex AB5 toxin, comprised of the enzymatically active, [unreadable]A[unreadable] subunit (S1), and the binding, [unreadable]B[unreadable] oligomer (PTX-B), composed of five subunits S2, S3, S4, and S5, found in a 1:1:2:1 ratio. The B portions of AB toxins transport the A subunit to the cytoplasm of target cells, and were not thought to participate in toxicity. However PTX-B has been shown to have activity in addition to its role in facilitating entry of S1 into the cytoplasm. PTX-B induces a spectrum of cellular responses, including immediate cell death, development of apoptosis, cellular clustering, and even cellular proliferation. In this proposal we will identify the receptors for PTX-B, we will characterize the signaling pathways that are affected by binding of PTX-B using T cells as a model system, and we will characterize its toxicity to other cells of the immune system. Specific Aim 1. Characterization of the PTX-B binding elements. PTX-B has been shown to bind to multiple receptors on cells. We will identify the essential PTX-B binding regions as a prelude to the identification of potential cell surface receptors. Specific Aim 2. Determine the mechanism by which PTX-B attenuates chemokine receptor signaling and chemotaxis. The mechanism by which PTX-B affects chemokine receptor activity remains largely unknown. We will extend our studies on the mechanism(s) of PTX-B signaling in T-cells to determine if PTX-B blocks lymphocyte migration by promoting chemokine receptor desensitization using the chemokine receptor CCR5 as a model system. Specific Aim 3. Characterize the ability of primary cells to respond to intact pertussis toxin and PTX-B. Published reports suggest that the responses to PTX-B are different between mice and humans. We propose to characterize the short term and long-term responses of human and murine leukocytes both intact pertussis toxin and PTX-B.